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Haemocyte activity of the third, fourth and fifth larval instars of Apis mellifera (L.) was observed following artificial injection with a sublethal doses of the bacterium, Paenibacillus larvae larvae (White) the causal agent of American foul brood disease, and compared with naturally infected larvae at different disease intensities (low, medium and high). Thirteen types of haemocytes were described in healthy larvae: prohaemocytes, granulocytes, eosinophil cells, oenocytes, plasmatocytes, spindle-shaped cells, micronucleocytes, macronucleocytes, spherulocytes, pycnonucleocytes, basophil cells, adipohaemocytes and neutrophil cells. Plasmatocytes represent 60-90% of the total haemocyte population and the other cells represent 10-40%. No marked changes were observed in percentages of plasmatocytes at all examination periods post-infection in all larval stages. While, in naturally infected larvae, their percentages increased significantly at all intensities of infection. In total haemocyte counts (THCs), there was a significant increase in the third larval instar, and a significant decrease in the fourth and fifth larval instars was observed at all examination periods. In naturally infected larvae, a significant decrease in THCs was observed at all intensities of the disease. Phagocytic response was observed in all tested instars, the maximum rate was recorded at 24 h post-injection. Percentages of phagocytosis increase as the larval age increases. Also, in naturally infected larvae, phagocytosis was observed at all intensities of infection, the maximum rate was at the high intensity of infection. Injection of P. l. larvae caused a significant increase in nodule formation in third and fourth larval instars. In contrast, a significant decrease in nodule formation in fifth larval instar was observed. Nodule formation was not determined in naturally infected larvae due to the extensive destruction of bee colonies.

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Haemocyte Activity and Cellular Defense Reactions in Various Larval Instars of Honey Bee (Apis mellifera L.) following Natural and Experimental Bacterial Infections